cryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

Thomas Dieingi, Patrick Altmannii, Jan Englerti, Damon Stromi,
Mirko Bacaniii
i WITec GmbH, Ulm, Germany (info@witec.de)ii attocube systems AG, Haar, Germany (info@attocube.com)

1. Introduction© attocube systems AG & WITec GmbH, 2021. All rights reserved.

Interest in Raman spectroscopy at room temperature, as well
as at cryogenic temperatures, has been increasing across a
range of scientific fields over the past two decades [1,2,3].
Initially, the primary driver of cryogenic applications was the
carbon nanotubes and graphene community [4,5,6]. As a
response to the needs of these researchers, attocube systems
and WITec – world leaders in their respective specialties ofcryogenic scanning probe microscopy and Raman spectroscopic
imaging – teamed up in 2010 to develop attoRAMAN, theworld´s first commercial cryogenic Raman microscope.
In the meantime, cryogenic Raman spectroscopy in highmagnetic fields has become an indispensable tool for studying
various novel materials, in particular for researchers focused onphase-transitions or emergent properties of low-dimensional
materials with application potential in electronics or sensorics.
A recent surge in the number of published cryogenic Raman
studies is dominated by research on transition metal
dichalcogenides and van der Waals heterostructures[7,8,9,10,11]. In order to meet evolving market requirements,
the two southern German companies combined their expertiseonce again to create cryoRaman - the cutting-edge solution for
cryogenic Raman spectroscopy.
Building on the established success [12,13,14,15,16,17,18,19]
of previous WITec and attocube co-development projects andfurthering the integration of the constituent technologies
enables cryoRaman to deliver improved accessibility andconfigurability. Several new functionalities, such as lowwavenumber Raman peak detection, polarization control andthe full automation of measurements, render cryoRaman
tunable to specific needs of different research fields. Moreover,as the combination of superior optics and the lowest-vibration
cryostats, cryoRaman offers the highest data acquisition speedwith unequalled lateral, depth and spectral resolution.

2. Description
instrument

of

the

cryoRaman

The fundamental advance with respect to existing products isthat cryoRaman can be equipped with the full catalog of WITec
optical components as known from their alpha300 confocal
microscope series. This not only delivers a significant leap in
1

spatial resolution (<400nm laterally, <2µm in depth), it alsomeans that cryogenic confocal Raman microscopy can now be
augmented with a wide range of upgrades and capabilities thatthe community has been requesting. For instance, with the
RayShield coupler, wavenumbers down to 10 rel. 1/cm and antiStokes signals are now easily detectable and this approach canbe combined with the polarization upgrade, which isparticularly interesting for the field of valleytronics [20]. Other
modules that enhance the system’s flexibility and convenience
have also found their way into the list of available components:
automated alignment and calibration, TruePower (automatedlaser output adjustment) and motorized switching between
different excitation lasers and white light microscopy. With
these functionalities cryoRaman has achieved a level of userfriendliness that makes high resolution confocal Raman imaging
at cryogenic temperatures in high magnetic fields accessible toresearchers of all experience levels.
The unique advantages of cryoRaman stem from the seamlessintegration of WITec’s high-throughput optics and leading-edge
Raman technology with the exceptional performance andefficiency of attocube’s closed-cycle cryostats. The
attoDRY2100 series is a high-end cryogenic system compatible
with cryoRaman that allows for measurements at variabletemperatures from <1.8K up to 300K. With a wide range ofsolenoid and vector magnet options, it is particularly well suited
for spin-resolved spectroscopy and can be complemented by insitu magneto-transport measurements with the addition of
coaxial lines. If only the base temperature is of interest, theattoDRY1000 offers an affordable alternative. Much research in
the field of quantum optics and (time-resolved) valleytronics
requires complex optical setups. These can generally berealized with WITec optical components, but for additional
flexibility, the integration of cryoRaman with the attoDRY800
cryostat [21] is currently under development.
The outstanding performance of cryoRaman is only achievableby preserving the full capabilities of both companies’
technology in combination. It is often difficult to achieve highconfocality within a cryogenic system because the bore size ofthe magnet leaves little space and optical access is hindered. A
proven solution to this problem is found in attocube’s lowtemperature objectives. The LT-APO objectives are designed
for, and tested at, low temperatures and high magnetic fields.
Unique in the marketplace, they offer outstanding optical
performance, numerical apertures of 0.81 - 0.82, and theirapochromatic ranges match all typical Raman laser wavelengths(532nm, 633nm, 785nm) while enabling a lateral resolution of<400nm in confocal scanning.
High resolution Raman map creation is facilitated by a directelectronic interface between WITec’s scan controller andattocube’s cryogenic piezo scanners. This makes it possible toprogram all typical Raman and photoluminescencemeasurements within WITec’s Suite FIVE control software. WithcryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

attocube’s patented slip-stick drive piezo steppers, the sample
can be brought into focus and the sample’s region of interest
can be accessed.
In its base configuration, cryoRaman features a 532nm
excitation laser and a RayLine coupler, which gives access towavenumbers of >80 rel. 1/cm. The attoDRY2100 with a 9T
solenoid magnet is the typically specified cryostat.
The following sections provide more details on available
upgrade options.

2.1 Volume Bragg gratings
In addition to Raman spectroscopy using conventional edge
filters (RayLine design), which allows for the detection of Stokes
scattering from typically ~+80 rel. 1/cm onwards, the use ofvolume Bragg gratings has become ever more popular in recent
years [22,23,24]. Using these optical elements, the detection of
Stokes as well as anti-Stokes Raman scattering is possible withthe additional benefit of detectability down to typically ±10 rel.
1/cm. In the WITec setup these elements are integrated within
the RayShield coupler and are available for most common
wavelengths [25].

© attocube systems AG & WITec GmbH, 2021. All rights reserved.

2.2 Polarization analysis
Irrespective of whether a RayShield or RayLine coupler is used,the excitation laser light always enters the microscope with ahighly linear polarization. Using the polarizer upgrade, thedirection of the linear polarization can be rotated, or it can beconverted into circular polarization. In addition, an analyzer canbe introduced into the detection beam path to allow only
certain polarization directions to pass through to thespectrometer. Both the polarizer and the analyzer can berotated independently and freely in order to match, forexample, the crystalline axis of a sample. Both polarizer andanalyzer modules are available in a manual as well as a
motorized configuration. The latter version allows theacquisition ofPolarization series (rotation of the incoming
polarization while the analyzer remains fixed andautomated recording of spectra at selectable angles)
Analyzer series (rotation of the analyzer while theincoming polarization remains fixed and automatedrecording of spectra at selectable angles)
Combined series (rotation of the polarization
direction and of the analyzer with a fixed relative
angle and automated recording of spectra atselectable angles)

2.3 Motorization and automation possibilities
Apart from the above-mentioned motorization of the polarizer
and analyzer, most other components used in the cryoRaman
setup can also be motorized. For WITec’s alpha300 and RISE
platforms, which use parts largely identical to those in2

cryoRaman, the most popular motorized feature is automatic
switching between white light microscopy and Raman
microscopy. This module greatly accelerates the experimental
workflow and eliminates the need to touch the system at all
during the most frequently performed operation on any
confocal Raman microscope. Switching between various
excitation wavelengths can also be automated and the recently
introduced AutoBeam technology automatically optimizes thepinhole position for each excitation laser independently of thesample. The AutoBeam coupler also automatically switches
between different spectrometers in, for example, systems
optimized for both VIS and NIR wavelengths. Another aid toefficient operation is a fully automated calibration coupler. In
combination with WITec’s software-driven multi-point
spectrometer calibration over the full spectral range, it ensuresa high level of confidence in the data recorded.

2.4 TruePower laser control
In addition to automation of the microscope components
themselves, the TruePower laser power control module
enables the precise setting and documentation of the laser
power used with an accuracy down to <0.1mW (<1µW uponrequest). With this option the laser always operates at the same
output power, which enhances its stability and lifetime, and thelight intensity transmitted to the microscope can be attenuatedthrough software. In addition to the precise setting of thepower and its automatic recording with each measurement,
TruePower allows the acquisition of power series. In thismeasurement mode, the laser power is varied and at eachpower level a spectrum is recorded. Through the corresponding
variation of the integration time, the spectra show
approximately the same intensity, but laser power-induced
peak shifts or sample modifications can easily be evaluated. The
researcher can then set the optimal laser power for eachsample. TruePower also includes an automatic shutter thatalways blocks the laser whenever a measurement isn’t being
performed. This avoids unnecessary exposure of the sample tolaser radiation and enhances the overall safety of the system.

2.5 Raman excitation and detection options
The cryoRaman system can be equipped with various excitation
wavelengths throughout the VIS to NIR spectral range. For
optimized detection the WITec UHTS spectrometer series [26]consists of several models. This allows the use of optimized
optics, gratings and CCD cameras for the corresponding
detection range, without the need to manually exchange
gratings or manually re-align any elements within the
spectrometer.
The optimal cryoRaman set up in terms of excitation
wavelength or spectrometer variant largely depends on
individual experimental requirements and many configurations
are available upon request.

cryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

3. High resolution, low temperature
confocal Raman imaging on WSe2

© attocube systems AG & WITec GmbH, 2021. All rights reserved.

As a first demonstration of cryoRaman’s confocal Raman
imaging capabilities, a WSe2 flake on Si/SiO2 was recorded at
120K. The image is 30x30µm² in size with 400x400 pixels(spectra) resolution and an integration time of 150ms per
spectrum. A 532nm laser was used for excitation and the
RayShield coupler allowed detection of Raman bands down to~±10 rel. 1/cm. A UHTS300 spectrometer optimized for
detection in the visible range with an 1800g/mm grating and aback-illuminated CCD camera was used.
Inside the cryostat an attocube LT-APO 532-RAMAN objectivewith a numerical aperture of 0.82 and attocube piezoelectric
positioners controlled through WITec’s Control FIVE software
were used. The acquired data set was evaluated with the WITec
Project Plus software package using the cosmic ray removal tool
as well as background subtraction before TrueComponent
Analysis was employed to automatically identify the sample
components (Figure 1[b]) by layer number. The images showing
the tungsten diselenide in different numbers of layers werethen combined in the false color image shown in figure 1[a].
Figure 1[c] shows a zoom-in of the low-wavenumber area withthe most prominent peaks for the double and triple layers aslabelled, which is in good agreement with the literature [27].

The presence of the single, double and triple layers could beclearly identified by the peaks present. The unknown phaseshows in part the features of the single layer, but with anenhanced intensity of the Si-peak near 520 rel. 1/cm.

4. Spatial resolution of cryoRaman
In order to determine the resolving power of the system at low
temperatures, a sample with oriented carbon nanotubes (CNTs)
on Si was investigated in a confocal Raman imaging scan at 2K.
Here an area of 29x29µm² with 290x290 points was scanned
using a RayLine Raman coupler (only allowing Stokes detection)with the remaining parameters as described above. Figure 2[a]is the image of the integrated intensity of the CNT G-band while
Figure 2[d] shows a typical CNT Raman spectrum. In Figure 2[b,c] two extracted cross sections depict the determined full widthhalf maxima (FWHM) of the CNTs.
As the ring-breathing modes (RBMs) are detectable, it can beconcluded that the measured CNTs were single-walled. They
were therefore much smaller in diameter than the limit ofresolution of the instrument. Mathematically speaking, they
represent a delta-function for the convolution of sample
features with the instrument function. Thus, the measured

Figure 1: Confocal Raman image of WSe2 of various layer numbers at 120K. [a] Combined false-color
image showing the location of the different numbers of layers. [b] Raman spectra of the layers in thecorresponding colors and [c] zoom-in of the spectra around the Rayleigh peak.

3

cryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

Figure 2: Confocal Raman imaging scan of CNTs at 2K. [a] Integrated intensity of the CNT G-Band. [b,c] Cross section of the intensity profile of the recorded CNTs. [d] Typical spectrum of one of the CNTs
showing the spectral features of the CNT and the underlying substrate.

© attocube systems AG & WITec GmbH, 2021. All rights reserved.

FWHM can be taken as a measure for the achievable lateral
resolution with this system (see also [28]).
The depth resolution of the system was determined with a plain
piece of silicon as being ~1.7µm in the above-mentioned
configuration. It has been shown in literature [28] that thedetermination of depth resolution using Si produces results
similar to those on suspended graphene at 532nm excitation,
which would again represent a delta function for the
convolution.

5. Temperature-dependent PL
measurements on WSe2
In
order
to
demonstratetemperature-dependent
photoluminescence (PL) measurements, part of the same WSe2
flake as shown above was measured at varying temperatures.
For this experiment, the photoluminescence signal wasrecorded instead of the Raman signal. The same hardware as

described above was used, but the 600g/mm grating was
rotated under software control to a slightly different angle forthis purpose. Figure 3[a-d] shows the position of the PL peak of
the WSe2 recorded at different temperatures, while Figure 3[e]shows example spectra recorded from the flake and the edgeof the flake for each temperature. The two- and three-layer
areas of the flake did not emit significant signals, as was
expected. It can be seen that the PL peak near the edge isshifted to lower wavelengths (higher energies) at all
temperatures. Such a shift in the photoluminescence peaks of
WSe2 flakes has already been reported for room temperatureexperiments [29]. Figure 3[e] additionally illustrates the peakshift as well as the peak sharpening with decreasing
temperature.

6. Polarization analysis of WSe2
The same sample position as used for the PL measurement
above was also scanned at 2K through varying polarizer andFigure 3: Confocal photoluminescence imaging scans of WSe2 at varying temperatures. [a-d] Peak position
images at varied temperatures. [e] Normalized spectra at each recorded temperature with the solid lines
representing the spectra in the center of the single layers and the dashed lines the spectra at the outer edge ofthe single layers, respectively.

4cryoRaman

Cutting-edge Cryogenic Raman Microscope© attocube systems AG & WITec GmbH, 2021. All rights reserved.

white paper

Figure 4: Polarization-dependent spectra of double and triple layer
WSe2 at 2K. All spectra are normalized to the peak near 395 rel.1/cm.

Figure 5: Polarization-dependent spectra of single-layer MoS2
recorded at 9T and 2K.

analyzer orientations. The Raman peak near 250 rel. 1/cm (seeFigure 1[b]) was first iteratively optimized by rotating thepolarizer and analyzer to determine the parallel polarization
with respect to the crystalline orientation. A Raman image wasthen recorded in this configuration before the analyzer was
rotated by 90° (optimal perpendicular configuration) and thescan was repeated. From the resulting data sets, the average
spectra for the corresponding layers were extracted as shownin Figure 4.

measurement in the perpendicular configuration the analyzer
was then rotated by 90°. Figure 5 shows two example spectra
recorded in this configuration while a magnetic field of 9T wasapplied perpendicularly to the sample surface.

The spectra were normalized to the peak near 395 rel. 1/cm inorder to facilitate the comparison. It can be seen that the Si
peak near 520 rel. 1/cm is almost completely suppressed. For
both the double and the triple layers, the peak near 305 rel.
1/cm is also largely suppressed in the perpendicularconfiguration. The fact that it was not completely suppressed
may be due to the averaging process, by which imperfectlocations on the layers were also taken into account. For the
double layer, it can additionally be seen that the peak near 27
rel. 1/cm is largely suppressed in the perpendicularconfiguration, whereas the peak near 17 rel. 1/cm appears
stronger in comparison.

7. Magnetic field- and polarizationdependent measurements on MoS2
In order to demonstrate the dependence of Raman signals onpolarization in varying magnetic fields, experiments were
performed following the examples shown by Yang et al. [30]and Ji et al. [31]. For this a MoS2 flake on Si was imaged at <2K
while both the magnetic field as well as the polarization wasvaried. The location chosen on the sample contained a flake
consisting of different numbers of layers. The optimal parallel
configuration for the polarizer and analyzer was determined by
iteratively rotating the polarizer and analyzer until the 𝐴1′
Raman signal of a single layer of MoS2 was maximized. For the5

The suppression of the 𝐴1′ Raman signal of MoS2 can clearly beseen in the perpendicular configuration. In addition, the 1storder Si peak was reduced in intensity, but is still significantly
stronger than the MoS2 peaks. This is in contrast to the spectra
shown in Figure 4 and can be attributed to the fact that theorientation of the MoS2 flake relative to the Si crystalline axis
was different than the orientation of the WSe2 flake on its
substrate. The 2nd order Si peak, however, was largely
suppressed in the perpendicular configuration for the given
orientation (see Figure 5).
The flake was imaged using 532nm excitation (8mW before theobjective) through a RayLine filter set and a UHTS300
spectrometer with a 1800g/mm grating and a back-illuminated
CCD camera. The objective was again a LT-APO 532-RAMAN
objective with a numerical aperture of 0.82. Images wererecorded with an image size of 28x22µm² with 112x88 points
per image and an integration time of 0.15s per spectrum.
Images using identical parameters were recorded in varying
magnetic fields and in parallel and perpendicular polarization
configurations, respectively. The integrated intensity of the 𝐴1′
Raman signal of MoS2 was normalized against the integrated
intensity of the 𝐸 ′ peak for each measurement and all images
in Figure 6 are coded using the same color scale. Pixels at which
no 𝐸 ′ peak was present were suppressed in the visualization
below to enhance clarity and the lower limit of the color scale
was intentionally set to -0.1 in order to visualize the flake evenin configurations where no 𝐴1′ Raman signal was detectable(dark gray areas).
The change in suppression of the 𝐴1′ Raman signal fromperpendicular to parallel polarization configurations as themagnetic field is increased from 0 to 5T, and back again whenapproaching 9T, is in good agreement with [30]. However, incryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

Figure 6: 𝑨′𝟏 Τ𝑬′ intensity ratio recorded at 2K for different magnetic field strengths and polarization states.

© attocube systems AG & WITec GmbH, 2021. All rights reserved.

[31] this change could only be detected at significantly higher
magnetic fields. Yang et al. [30] propose the low temperatureused in their experiments as a potential explanation, which was
also the case in our experiments.
While influence of the perpendicular magnetic field on theoscillation modes of the crystal seems plausible, it is important
to consider the impact of the Faraday effect. The Faraday effectcauses a rotation of the polarization axis when light travels
through transparent media in the presence of a magnetic field.
In practical terms this means that the polarization axis of thelight is rotated after passing through the LT-APO objective by anangle that correlates linearly with the magnetic field strength.
To further illustrate this, the relative suppression of the 𝐴1′
mode in MoS2 is compared to the relative suppression of the 2ndorder Si peak.
For this purpose, a series of spectra was recorded on a singlelayer MoS2 flake in varying magnetic fields and in parallel andperpendicular configurations. The integrated intensity of the
MoS2 𝐴1′ Raman signal was again normalized against the
integrated intensity of the 𝐸 ′ peak. In addition, the 2nd order Si

Figure 7: 𝑨′𝟏 Τ𝑬′ of MoS2 ratio and Si 2nd/1st order peak ratio recorded
at 2K for different magnetic field strengths and polarization states.

6

peak was normalized against the 1st order Si peak. Figure 7
shows the resulting data as a function of the magnetic fieldstrength in parallel and perpendicular configurations. The data
for each respective data set was normalized from minimum tomaximum.
The MoS2 and Si peak ratios show approximately the samedependence on the magnetic field. This indicates that thedominant cause for the changes in the detected Raman signals
is the rotation of the polarization axis induced by the Faraday
effect. Previous tests at attocube have demonstrated that therotation angle is predictive and highly linear with respect to themagnetic field strength. It is therefore straightforward tocompensate for this rotation by adjusting the incoming
polarization axis accordingly. In combination with WITec’s
motorized optical components, this compensation could evenbe automated e.g. via LabVIEW-driven control of both
instruments.

8. Conclusion
As demonstrated conclusively in the measurements above,
cryoRaman offers a tremendously wide range of experimental
possibilities while maintaining a user-friendly mode ofoperation. In the attoDRY2100 cryostat the sample basetemperature is <1.8K and is variable up to 300K. Within thisrange of temperatures, Raman and photoluminescence peaks
can be observed, and their temperature-dependent shifts andchanges are of great interest in the study of phase transitions.
High resolution Raman maps with a lateral resolution of<400nm not only allow for material characterization, but also
for selectively identifying regions of interest. Raman peaks
down to 10 rel. 1/cm are conveniently detected andpolarization control offers many possibilities in the field of spinvalley physics. This is complemented with unidirectional andvector magnet options. A wide variety of combined in situ

cryoRaman

Cutting-edge Cryogenic Raman Microscope
white paper

measurements becomes possible by the introduction of coaxial
and optical fiber feedthrough lines.
In addition to the examples shown in this whitepaper, theextensive catalog of confocal measurement techniques
available with the WITec alpha300 series can also be
implemented in the cryoRaman platform, such as timecorrelated single photon counting (TCSPC) techniques (e.g.
FLIM) or antibunching measurements.

© attocube systems AG & WITec GmbH, 2021. All rights reserved.

WITec and attocube hope that cryoRaman will be instrumental
in many important scientific discoveries to come. We havedesigned it to serve that purpose. It offers the highest spatial

resolution on the market (400nm), sharpest depth resolution(2µm), lowest cut-off (10 rel. 1/cm), cryogenic objectives withhighest NA (0.82), lowest z-vibrations (1nm peak to peak), mostaccurate power control (0.1 mW), highest spectral resolution (<0.8 rel. 1/cm), lowest temperature (1.8K), highest magnetic
field (12T and vector fields), and highest data acquisition speed
enabled by the highest sensitivity (<1ms per spectrum).
Moreover, cryoRaman - in its modularity - offers upgrade andmotorization possibilities that provide for a unique userfriendly experience that meets the individual requirements ofspecific applications.

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